2 and 4 exist for non-windows platform, and are pretty stable. 1 is the most stable for me (i'm using windows), 3 is unreliable, you can get a lot of weird messages like "can't co-create object" (it uses JACOB under the hood, see http://sourceforge.net/projects/jacob-project/ )
As far as I know, 3 and 4 are evolutions of 1 and 2. ClearcasePlugin and eclipse-ccase plugin writers just decided to put in common re-usable parts of their respective plugin.

The reliable implementations already works well for basic functionalities :

update

checkin & checkout & undo checkout files

check in project

rename class (make sure you use checkin-project though)

delete class (requires you to checkout the file first... odd)

.. and not so well for advanced ones :

lack of activities support, although i've implemented a version for implementation number ... 3, the unreliable one

lack of synchronization feature to obtain correct clearcase status for each files (you must make sure you haven't any checkout and your view is up-to-date, then you 'mark your project as current')